reliability in all three directions (X,Y 
and 2). 
To achieve optimum angles of intersection it 
becomes necessary to take photographs such that 
the camera axes themselves are steeply inclined 
relative to one another, but yet pointed toward 
the object. (This technique is basically the 
same as increasing the length of a base line from 
which horizontal and vertical angles are turned 
with a transit or theodolite.) Photographs taken 
in this manner are said to be convergent because 
their axes converge. This is in sharp contrast to 
the usual case of stereo photogrammetry wherein the 
camera axes are purposely made nominally parallel. 
Multiple Photographs 
  
As alluded to above, more than two rays can be 
mathematically intersected at a given point of 
interest. Hence, more than two photographs can 
be taken, measured and introduced into the computer 
program; the larger number of intersecting rays 
again increases the reliability of the intersection. 
These additional photographs are ideally taken 
from totally different exposure stations, thereby 
providing even greater uniformity of reliability 
of the intersection in all three directions. This 
can be visualized with Figure 1 if one were to 
imagine a third photograph behind the plane of the 
sketch and a fourth in front.